A database of words such as a synonym dictionary obtained by collecting words having high relativeness is used for supplement of a search keyword and increase in precision of translation and the like in an information search system, a natural language processing system, a machine translation system, and the like. Various devices for automatically generating words having high relativeness and using them for a search and the like are being studied. For example, Patent Document 1 discloses a keyword searching device which obtains a character string of an input keyword, extracts a comment including the keyword, extracts only nouns from the words by performing morphological analysis process, divides a Japanese sentence by words, extracts the word with word class, generates a word histogram for organizing the extracted words in descending order of the number of use times, assures words whose number of use times is large as a related keyword, and re-retrieves a comment on the basis of the related keyword.